Closing the circle: how ecologists can prepare their own quality control material to increase confidence in stable isotope data


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  1. Stable isotope analysis (SIA) is widely used in ecology but is usually performed by specialist facilities. Consequently, the reliability of SIA data depends completely on the facility's quality systems unless ecologists take steps themselves. Although widely recommended, practical advice on how to do this is lacking.
  2. Here, we outline preparation of 150 vials of an in-house quality control material (QCM) based on air-dried soil as an exemplar and describe methods of data analysis applicable to any QCM matrix. Homogeneity testing of the candidate material involves duplicate analysis of 10 randomly chosen vials, a Cochran test for outliers and 1-way anova. Given that many ecologists find SIA expensive, we propose that an ecologist preparing a soil QCM pretest for homogeneity using a simple colorimetric analysis of a soil quantity (bicarbonate-extractable P) sensitive to inhomogeneity.
  3. Once pretest homogeneity has been established, homogeneity under SIA can be tested. We obtained a 1-way anova P value of 0·824 in our laboratory pretest, then 0·782 (δ13C) and 0·125 (δ15N). QCM stability should also be confirmed.
  4. Submission of each batch of samples for SIA should be accompanied by a vial of the QCM, labelled as a normal sample. Control charting of the results then allows detection of within-batch issues encompassing any component of the analytical process (transport to the laboratory, sample preparation and subsampling, sample analysis, results calculation and reporting) and assessment of long-term analytical control. Any issue thus identified can form the basis of a conversation with the SIA laboratory about the affected batch of samples.